Spinning modes in orderly jet structure and jet noise
The structural response of a subsonic turbulent jet to controlled forcing has been investigated both theoretically and experimentally. An experimental study has been carried out in which a model jet was subjected to acoustic excitation in the form of circumferential modes of azimuthal wavenumber m=0,1 and 2. Detailed aerodynamic measurements on the initial region of the jet indicate that the m=0 and m=1 modes have comparable axial growth rates. Further, when forcing was applied in a combination of first (m = ±1) or second (m = ±2) order modes, substantial changes in the mean flow were evident over the first twelve diameters of the jet. A linear stability analysis of higher order azimuthally coherent wave-like disturbances on a slowly diverging mean flow has been used to predict the axial-growth rates, phase speeds and radial distributions of the flow characteristics in the forced turbulent jet. Similar axial growth rates result for the m=0 and m=1 modes but, in both these cases, they compare unfavourably with the measured rates which reflect the highly non-linear response of the instability wave to the strong forcing level used. The phase speeds are well predicted, as are the measured radial structures of the instability wave velocity components. The latter agree surprisingly well in view of the highly non-linear axial development of these quantities and this interesting result lends support to the previously unwarranted use of "shape functions" in non-linear stability calculations. Finally the effects of an external flow on a forced jet have been considered. Linear stability theory has been used to examine the axial development of a plane wave disturbance on a slowly diverging jet embedded in an infinite co-flowing stream. The results indicate that at a diameter-based Strouhal number of 0.5 the disturbance suffers very little downstream amplification in the presence of a significant external flow. This is in agreement with increasing experimental evidence which casts doubt on the significance of jet forcing when the device producing the jet is in flight relative to the ambient medium.